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The oldest rodent fossils yet found in South America have been unearthed along the Ucayali River near Contamana, Peru.

The specimens comprise the tiny teeth of mouse-sized and rat-sized animals that lived at least 41 million years ago.

This makes the fossils some 10 million years older than all previous rodent discoveries made on the continent.

An international team of scientists reports the remains in the Royal Society journal Proceedings B.

The researchers describe how the shape of the teeth and other factors point to the ancient animals being most closely related to African rodents.

"As palaeontologists, we're interested in how animals are related to each other, and we do what are called 'phylogenetic analyses'," explained co-author Darin Croft, an anatomy professor at Case Western Reserve University School of Medicine, Cleveland, US.

"We did those analyses for our animals and they are very close in the evolutionary tree to African rodents, which suggests that that's where their ancestors came from - from Africa," he told BBC News.

Tiny teeth

Dr Croft and colleagues had to use microscopes to study the teeth, so small were some of them.

Features seen in the dentition suggest the animals probably ate soft seeds and plant parts, just like many small rodents do today.

Teeth seen against human fingers

Pollen found with the teeth indicates these rodents would have scurried around in a similar rainforest to the one that lines the modern Ucayali River.

The team has assigned some of the fossils to new species. All were caviomorphs, the same taxonomic group that includes some of South America's iconic rodents today - including guinea pigs, chinchillas, and capybaras, which at 60-70kg are the world's biggest.

The scientists are very sure of their timings because the fossils were found just below a layer of ash that could be very accurately dated through a well established technique that compares the sediment's ratio of different types, or isotopes, of the element argon.

This agrees well with genetic studies of modern African and South American rodents that show the animals are of common origin, and estimate they arrived in South America during what is termed the Mid-Eocene Climatic Optimum - a very warm phase in Earth history.

Pollen in the sediments indicates the rodents lived in a rainforest environment - much like today

No-one really knows how they got there, but scientists have speculated that some small animals could have made the journey by sea.

"They could have got there on some raft of vegetation," said Dr Croft.

"That maybe sounds like a fantastic tale, but in fact we do see things like this happening today. You can get big logjams of vegetation that get pushed out of rivers during storms, and often you will see mammals on them.

"The odds of them making this crossing are obviously very low, but after millions and millions of years the odds of some animals making it go up considerably.

"And if we go back to the middle of the Eocene when we think rodents might have made this crossing, the two continents of South America and Africa were actually closer together than they are today - about half the distance."

The study was led by Pierre-Olivier Antoine, a professor of palaeontology in the Institute of Evolutionary Sciences at Montpellier University, France.

BBC links

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